Christians should be able to answer this question because it is one asked by many unbelievers.  If you are unable to answer they will try to convince you that there were people on earth prior to Adam and Eve.  If you answer sisters, they will tell you that God forbid marrying close relatives.

Genesis 2:20 – “So the man gave names to all the livestock, the birds in the sky and all the wild animals. But for Adam no suitable helper was found.”  This tells us there were no humans but Adam at that time.  God, taking a rib from Adam, made Eve, the woman.  If there were other women He would not have had to create Eve.  He could have put His Spirit into her if another existed to make a suitable helper.

Genesis 1:28 – “God blessed them and said to them, ‘Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground.’” God told them to fill the earth with children.  Jewish historians say they had over 30 sons and over 30 daughters.

Genesis 5:3 – “When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth.”  We are told that Adam lived to be 930 years old (Gen 5:5).  God made Adam and Eve with perfect genes.  They were able to live long lives and have children with different genes than humans have today. Abraham married Sarah, who was his half-sister, (Gen. 20:12).  Close relatives could marry without children being born deformed.  It wasn’t until Moses generation did God forbid the marrying of close relatives.  This can be found in Leviticus chapters 18 thru 20.  The human genes had deteriorated from Adam’s time and no longer could people live hundreds of years and marry close relatives.  Usually they had deformed and unhealthy children.

Those first brothers and sisters did not practice birth control.  It would not be unusual to have ten or fifteen children each or as Adam and Eve, 60.  Over a period of 900 years you could have thousands of people living.  By the time Moses lived, there were millions on earth.  There were all kinds of races and religions.  Pagan religions practiced sexual relations with closes relatives, they committed adultery, children were sacrificed and some had sexual relations with animals.  God taught that these were unacceptable abominations or wicked actions and were banned and God dealt with them harshly.    These practices led to disease, deformity, death plus social issues.  Therefore, God instituted laws forbidding the Israelites to participate in this behavior.

Over the past 6,000 years, living under God’s curse and the evil practices of pagan religions, we probably are less intelligent, less healthy and see the world much differently than the early families.  However, we are told that Christ will come to rule for 1,000 years on this earth.  Christians will be given immortal bodies and be able to live as Adam lived, close to a thousand years.  During that time there will be children born and people will have a choice to follow Jesus and some will be deceived by Satan when he is released from the abyss according to the Book of Revelation.

So if anyone asks who were Cain and Abel’s wives you can answer them.  Studying Genesis will give you a picture of how it all started and you can use it to defend the Gospel and those who are unbelievers and question the Bible’s authenticity.